Single-sensor microcontroller-based approach for ground fault circuit interrupters

ABSTRACT

A microcontroller-based temperature compensated circuit for ground-fault circuit interrupter to meet the requirements of UL 943 using a single sensor to detect both ground-fault and grounded-neutral fault conditions in both full-wave and half-wave AC power supplies as part of a ground-fault circuit breaker or a receptacle device.

BACKGROUND OF THE INVENTION

Existing designs for ground-fault protection devices such as circuitbreakers and receptacles typically use an analog circuit and two currentsensors to meet the requirements of UL 943. One sensor is needed fordetecting the current imbalance characteristic of a ground-fault, and asecond sensor is used as part of a dormant oscillator circuit fordetecting a grounded-neutral condition that can degrade the ground-faultdetection ability. These sensors are required to be of high precisionover a wide range of temperatures and to have low part-to-part variancesince the analog circuit offers little compensation or calibrationabilities. Additionally, the analog approach may not work well if thesupply is discontinuous since no non-volatile memory function isavailable.

SUMMARY OF THE INVENTION

Briefly, the present invention uses the combination of a single low-costcurrent sensor and a small, low-cost microcontroller, designed for useas part of a ground-fault circuit breaker or receptacle device to meetall the requirements of UL 943 while addressing the issues of existingdesigns.

According to another embodiment of the invention, the cost is reducedcompared to the two-sensor approach by combining the functions ofground-fault detection and grounded-neutral detection into one sensor.

According to yet another embodiment of the invention, a simpletemperature measurement and compensation scheme to correct for sensornon-linearities over temperature allows the sensor to be designed toutilize low cost materials and a simple manufacturing process.

Another embodiment of the present invention uses a programmable devicethat provides for software-based calibration during the electronicassembly process to overcome part-to-part variance in the sensorcircuitry. This allows for a wider acceptable tolerance range for thesensor circuit components and reduces the amount of rejected componentmaterial.

According to another embodiment of the present invention, an analogmemory function is provided to resume a circuit trip condition on adetected fault if power is temporarily lost before the trip circuit hastime to activate. This feature allows the circuit of the presentinvention to operate from a half-wave-rectified or other discontinuouspower source.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic diagram of a ground fault circuit interrupterembodying the invention,

FIG. 2 is timing diagram illustrating the use of the memory capacitor inthe circuit of FIG. 1,

FIG. 3 is a series of waveforms illustrating ground-fault detection witha half-wave power supply,

FIG. 4 is a series of waveforms illustrating ground-fault detection witha full-wave power supply,

FIGS. 5 a and 5 b are illustrating the detection of a condition where nogrounded-neutral exists, and

FIGS. 6 a and 6 b are illustrating the detection of a grounded-neutralcondition.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

Referring now to the drawings, and initially to FIG. 1, a digital,microcontroller-based ground-fault circuit interrupter (GFCI) circuit 10is arranged to sense ground-fault and grounded-neutral conditions online and neutral conductors 30 and 32, respectively, using a singlecurrent transformer T1 as a sensor.

The digital microcontroller U1 is a device such as the PIC12CE673microntroller, or a digital signal processor or an ASIC device withfeatures such as: on-board RAM, a non-volatile memory, an internaltimer, an internal analog-to-digital (A/D) converter and analog anddigital ports.

DC power for the GFCI circuit 10 is supplied from a power supply circuit20, which draws power from the line and neutral conductors 30 and 32,and a reference circuit 22 that produces the required regulated DCvoltage levels. The complete power supply consists of a trip solenoidL1, a varistor MOV1, a rectifier CR1, a capacitor C1 a dropping resistorR1, a diode string CR2-CR5, a reference diode CR6, and an outputcapacitor C3. The trip solenoid L1, the capacitor C1 and the varistorMOV1 perform input filtering and surge limiting. The trip solenoid L1serves multiple functions by providing input filtering, i.e., a seriesimpedance for surge and noise suppression, as well as a means to openthe main contacts (not shown) on a short circuit failure in the powersupply or for the intended trip function in case of a detectedground-fault or grounded-neutral condition. The rectifier CR1 rectifiesthe incoming AC current, and the capacitor C1 provides additional energystorage and suppression of high frequency transients. Thevoltage-dropping resistor R1 is sized appropriately to maintain enoughcurrent to forward bias the diode string CR2-CR6 in the voltagereference circuit 22, plus provide the required operating current forthe circuit at the minimum input voltage. The DC voltage level requiredfor operation of the microcontroller and other circuitry is regulated bythe diode string CR2-CR6. A reference voltage needed for stableoperation of the sensing circuit 24 is provided by CR6 and C3. Thecapacitor C3 provides a small amount of energy storage under transientconditions. The regulated Vref output is available over an input rangeof ˜66 to ˜132 VAC. The output voltage Vref and input range can beadjusted by changing component values, as is well understood by thoseskilled in the art.

A capacitor C2 and a Silicon Controlled Rectifier (SCR) Q1 perform atrip function. When a fault is detected by the microcontroller U1, thedigital output “trip” pin of the microcontroller U1 is set, which turnson the SCR Q1 and creates a current path through the solenoid L1, therectifier CR1 and the SCR Q1. The resulting current is at a levelsufficient to activate the trip solenoid L1 and open the main contacts(not shown). The capacitor C2 provides noise suppression for the gate ofSCR Q1 and stores voltage during the trip operation to maintain the “on”state of Q1 for a longer period of time.

A manual test circuit 18 consists of a manual push-to-test switch PTTand a pair of resistors R11 and R12. When the switch PTT is depressed, asufficient current flow occurs to cause the GFCI circuit 10 to detect afault and use the trip function to open the main contacts (not shown).

The current-sensing circuit 24 consists of a current transformer T1coupled to a line conductor 30 and a neutral conductor 32 and anamplifier circuit composed of an operational amplifier U2 and a pair ofresistors R7 and R8. A bias voltage resistive divider circuit formed bya pair of resistors R3 and R4, which sets up a circuit voltage that is ½of Vref. This assures that the “Zero” level of the sensor circuit 24output sits half way between the rails of the A/D input of themicrocontroller U1 to facilitate envelope detection.

The permeability of the current transformer T1 is affected by changes inenvironmental temperature which are preferably compensated for in bothground-fault and grounded-neutral threshold levels.

An optional, temperature-sensing circuit 26 uses the base-to-emittervoltage of a small-signal bipolar junction transistor Q3 to provide areading of the ambient temperature conditions near the currenttransformer T1. The junction bias current of the transistor Q3 is set bya resistor R13 connected to the reference supply voltage Vref. Thereference voltage Vref and the voltage at the base of the transistor Q3are sampled by the microcontroller U1, and the value sampled is used toadjust the ground-fault threshold value and grounded-neutral detectionreference value to compensate for changes in the performance of thecurrent transformer T1 over temperature.

During the manufacturing process, the microcontroller may be programmedto calculate the ground-fault and grounded-neutral threshold values at agiven temperature and store the threshold values in a non-volatilememory. Another temperature compensation method is discussed below withreference to FIGS. 5 and 6.

An analog, short-term memory circuit 28 consists of a capacitor C6, aload resistor R9 and a bleed resistor RIO. The microcontroller U1 uses abi-directional pin Mem_cap, as an analog input to read the voltage ofthe memory circuit 28 and as a digital output to charge the capacitor C6of the memory circuit 28. If a fault is detected, software running inthe microcontroller U1 causes a charge to be placed on the capacitor C6.If power is lost before the trip solenoid is able to open the contacts,the trip memory (i.e., voltage on the capacitor C6) will remain for ashort time and cause reactivation of the trip function (by themicrocontroller U1) upon resumption of supply voltage. The memorycircuit 28 allows the GFCI circuit 10 to operate from ahalf-wave-rectified or other discontinuous power source.

Referring now to FIG. 2, the timing diagram shows the use of the analogmemory circuit 28 during normal operation (no fault detected), fortiming purposes to determine when to execute the grounded-neutral andground-fault checks. The memory circuit 28 allows the timing ofgrounded-neutral checks to remain consistent even if ahalf-wave-rectified (discontinuous) power supply is used. When thevoltage of the memory circuit reaches the near-discharged state, themicrocontroller U1 charges the capacitor C6 to a voltage level less thatthe amount required to indicate a pending trip, as discussed above andexecutes a continuous ground-fault detection mode during the timeinterval until the voltage capacitor C6 reaches the near-dischargedstate again. When the voltage of the memory circuit 28, sampled by themicrocontroller U1, reaches the near-discharged state, agrounded-neutral check is executed during the intervening time or spaceinterval. This cycle occurs a few times per second as illustrated, andcan be adjusted by varying the values of the memory capacitor C6 and thebleed resistor R10.

Turning now to FIG. 3, the operation of ground-fault detection frompower-on to a circuit trip based on a half-wave-rectified power supplyis illustrated. At 100 a the power supply starts up, and at 102 a themicrocontroller U1 is initialized and the memory capacitor C6 is read todetermine if an unfulfilled trip condition exists from a previous cycleas discussed above. At 104 the ground-fault sensing function turns theswitch Q2 on, placing the low-impedance burden resistor R6 in thecircuit across the secondary of T1. The operational amplifier U2amplifies the voltage across the resistor R6 to a level that allows 5 mAof ground-fault current to be read by the A/D converter on-board themicrocontroller U1. The results are compared in software to a referenceground-fault threshold value to determine if the trip threshold has beenexceeded, indicating a fault. If a fault does exist, then at 106 thememory capacitor C6 is charged to indicate a pending trip condition, andat 108 a the trip function is activated in an attempt to cause a circuittrip in the time remaining. However, at 110 the half-wave power supplyshuts down. At 100 b the power supply starts up again, and themicroprocessor U1 is re-initialized at 102 b, but the charge on memorycapacitor C6 indicates a pending trip condition, so the trip function isactivated at 108 b to cause an immediate circuit trip.

When powered continuously with a full-wave power supply, as in FIG. 4,circuit tripping may occur more quickly since power is available toactivate the trip function during the negative half line cycle as well.Using a full-wave power supply, the startup cycles 100 and 102 of FIG. 3are only performed once on powerup/reset, and are not shown in FIG. 4.During ground-fault sensing, the microcontroller U1 turns the switch Q2on, placing the low-impedance burden resistor R6 in the circuit acrossthe secondary of T1. The operational amplifier U2 amplifies this signalto a level that allows 5 mA of ground-fault current to be read by theA/D converter on-board the microcontroller U1. The results are comparedin software to a reference ground-fault threshold value to determine ifthe trip threshold has been exceeded. If a fault does exist, then at 106the memory capacitor is charged to indicate a pending trip condition,and at 108 the trip function is activated to cause an immediate circuittrip. In case the main line circuit is interrupted before the circuithas tripped, the memory function, for a short time, can aid inperforming the trip immediately upon restoration of power.

Turning now to FIGS. 5-6, waveforms of the output of the current sensecircuit 24 are illustrated for the operation of the grounded-neutraldetection function when no grounded-neutral condition exists and when a1-Ohm grounded-neutral is present, respectively.

A grounded-neutral detection mode is entered when the voltage on thememory capacitor C6 reaches the near-discharged state. This occurs whenthe circuit is first powered up and every few hundred milliseconds afterthat, as determined by the memory circuit 28 for both full-wave andhalf-wave power supplies. In a grounded-neutral sensing mode, the switchQ2 is turned off by the Ping output of the microcontroller U1, whichswitches the gate voltage of the switch Q2 from high to low andgenerates a disturbance on the secondary of the current transformer T1through capacitor C5. With R6 switched out of the circuit, the secondaryof the transformer T1 and the capacitor C4 are allowed to resonate witha small amount of damping provided by the high-impedance burden resistorR5, as shown in FIG. 5 b. A grounded-neutral condition changes theimpedance of the secondary winding of the transformer T1 and dampens theoscillations sharply, as shown in FIG. 6 b. The envelope or peak-to-peakamplitude of the damped oscillatory waveform as it changes with time isamplified by the operational amplifier U2 and measured by the A/D inputof the microcontroller U1 after a pre-set delay.

The peak-to-peak amplitude of the waveform, or envelope, measured by themicrocontroller U1 is compared to a stored threshold for agrounded-neutral condition. If the peak-to-peak amplitude is greaterthan the threshold, then the primary impedance is above thegrounded-neutral threshold level, e.g., >2.5 Ohms. In this case, thememory capacitor C6 is charged for the next timing interval, thelow-impedance burden resistor R6 is switched back into the circuit bythe switch Q2, and the software program starts checking for aground-fault condition. If the measured peak-to-peak amplitude is lessthan a grounded-neutral threshold value, then a grounded-neutralcondition exists, the memory capacitor C6 is charged to indicate apending trip condition and the trip function is activated. FIG. 6 billustrates the damping effect of a grounded-neutral 34 condition causedby a 1-Ohm grounded-neutral, which causes the envelope of theoscillatory is waveform to decay rapidly, as compared to the envelopeillustrated in FIG. 5 b, where there is not a grounded-neutralcondition.

The aforementioned damped oscillations can be expressed in the form ofan exponential equation multiplied by a sinusoid as follows:A sin (ωτ)×e ^(−αt)

‘A’ represents the initial amplitude of the sinusoid, ω represents thefrequency of oscillation, τ represents time, and α is the decay factor.This α is the combination of the elements that cause the oscillation todecay. The neutral-to-ground resistance is directly related to this α.As the neutral-to-ground resistance goes down, α increases, causing thedecay to be faster. In order to determine the presence of apredetermined value of neutral-to-ground resistance, this α parametercan be calculated or estimated by a number of methods. Each methodoffers benefits and compromises in terms of processing requirements andsusceptibility to noise. Once estimated, the estimate may be compared toa setpoint for detection of a grounded-neutral fault. Each of thefollowing methods can be implemented with only the positive, negative orboth or absolute value of the oscillation cycles. These methods aredescribed below:

Method 1: Envelope of Peaks—Observing that the form of the expressionthat describes the decaying oscillation contains a sinusoid and anexponential function, this method seeks to find the envelope exponentialfunction. The peaks of the oscillation are located by sampling thesignal at a high rate. This peak-to-peak amplitude can be measured todetermine the envelope of the waveform. The envelope measured at aspecific time from the start of the oscillatory waveform can then beused to measure the decay rate of the exponential function.

Method 2: Polynomial Envelope of Peaks—This method is like Method 1 butuses a second-order estimate of the function in the form y=Ax²+Bx+C. Ais used to estimate α. A multi-order polynomial could also be used.

Method 3: Linear Envelope Estimate—This method is also like Method 1except a linear fit of the peak values is found. The resulting slope ofthe best fit line is used to estimate α.

Method 4: Area of Cycles—This method is like Method 1 but uses anestimate of the area below the signal waveform instead of peak values.The resulting points are fit to a model. A parameter of this model isused to estimate α. This method could use an exponential, linear orpolynomial model as in methods 1, 2 or 3 above.

Method 5: Slope of Half Cycle—This method estimates the slope of theleading or tailing edge of a half cycle by measuring two or more points.The parameter-to-base decisions could be the slope of half cycle N whereN is 1, 2, 3, 4 . . .

Method 6: Function of Slope of Half Cycles—This method requirescalculation of the slope of M half cycles and then use of a parametersuch as the slope of the resulting M half cycle slopes.

Method 7: Threshold on the Slope of Half Cycles—This method requirescalculation of the slope of M half cycles and then using a threshold tocount the number of half cycles above a preselected threshold. Thenumber of half cycles with a slope above the threshold is used as thedecision parameter.

Method 8: Count Peaks Above a Threshold—A fixed number of half cycles ora fixed timer period is monitored. During this time, the number of halfcycles that cross above a preselected threshold is counted. A decisionparameter based on the number of peaks above the threshold is used.

According to another embodiment of the present invention, the effect oftemperature on the performance of the current transformer T1 can bedetermined, during grounded-neutral fault detection, by measuring thefrequency of the damped oscillatory waveform of the current transformerT1. By measuring the resonant frequency with a known value ofcapacitance, changes in frequency can be related directly to changes inthe inductance of the current transformer T1. A change in inductance isa direct indication of a change in permeability in the transformer corematerial and also relates to the output characteristics of the currenttransformer T1.

According to one embodiment of the present invention, themicrocontroller is programmed, during the manufacturing process at abaseline temperature, to initiate the production of a damped oscillatorywaveform to produce a reference frequency value, and store the referencefrequency value in non-volatile memory. The reference frequency valueobtained is directly related to the inductance of the currenttransformer T1 at a baseline temperature. During normal operation of thepresent invention, the reference frequency value is compared to anoperationally measured resonant frequency, to calculate modifiedground-fault and grounded-neutral threshold values for use in the faultdetection process. Thus, changes in the performance of the currenttransformer T1, over a temperature range, can be made by a resonantfrequency observation in lieu of the optional temperature-sensingcircuit 26.

While particular embodiments and applications of the present inventionhave been illustrated and described, it is to be understood that theinvention is not limited to the precise construction and compositionsdisclosed herein and that various modifications, changes, and variationsmay be apparent from the foregoing descriptions without departing fromthe spirit and scope of the invention as defined in the appended claims.

1. A microcontroller-based system for detecting ground-fault andgrounded-neutral conditions in an electrical power distribution systemhaving line and neutral conductors comprising: a sensor circuitcontaining a single current transformer producing an output signalresponsive to current flow in both the line and neutral conductors ofthe electrical power distribution system, a microcontroller receivingsaid sensor output signal and initiating the generation of a trip signalupon detection of said ground-fault or said grounded-neutral conditionin said power distribution system, said microcontroller being programmedto use said sensor output signal to detect ground-fault conditionsduring spaced time intervals, and use said sensor output signal todetect grounded-neutral condition during intervening time intervalsbetween said spaced time intervals, a circuit interrupter forinterrupting current flow in said power distribution system in responseto said trip signal, and an analog memory circuit operable with bothfull-wave and half-wave power supplies to provide a timing function tocontrol said spaced time intervals and said intervening time intervals,and a memory function set in response to detection of a ground-fault orgrounded-neutral condition to resume a circuit trip if power istemporarily lost before said circuit interrupter activates.
 2. Amicrocontroller-based system for detecting ground-fault andgrounded-neutral conditions in an electrical power distribution systemhaving line and neutral conductors comprising: a sensor circuitcontaining a single current transformer that varies non-linearly withtemperature, producing an output signal responsive to current flow inboth the line and neutral conductors of the electrical powerdistribution system, a microcontroller receiving said sensor outputsignal and initiating the generation of a trip signal upon detection ofsaid ground-fault or said grounded-neutral condition in said powerdistribution system, and a non-volatile memory associated with saidmicrocontroller, wherein said microcontroller is programmed duringmanufacture to receive said sensor output signal at a given temperature,and compute a predetermined ground-fault threshold value based on saidsensor output and store said predetermined ground-fault threshold valuein said non-volatile memory, and compute a predeterminedgrounded-neutral threshold value based on said sensor output and storesaid predetermined grounded-neutral threshold value in said non-volatilememory.
 3. A microcontroller-based system for detecting ground-fault andgrounded-neutral conditions in an electrical power distribution systemhaving line and neutral conductors comprising: a sensor circuitcontaining a single current transformer producing an output signalresponsive to current flow in both the line and neutral conductors ofthe electrical power distribution system, said current transformerhaving an inductance that varies with temperature. an ambienttemperature sensing circuit placed proximate to said currenttransformer, producing a voltage which varies linearly with ambienttemperature conditions, a programmable microcontroller having apre-determined ground-fault threshold value and a pre-determinedgrounded-neutral threshold value stored in a non-volatile memory, saidmicrocontroller being programmed to calculate a modified ground-faultthreshold value based on said predetermined ground-fault threshold valueand the output of said ambient temperature sensing circuit, calculate amodified grounded-neutral threshold value based on said predeterminedgrounded-neutral threshold value and the output of said ambienttemperature sensing circuit, use said modified ground-fault thresholdvalue to detect a ground-fault condition, use said modifiedgrounded-neutral threshold value to detect a grounded-neutral condition,and initiate the generation of a trip signal upon detection of saidground-fault or said grounded-neutral condition in said powerdistribution system, and a circuit interrupter for interrupting currentflow in said power distribution system in response to said trip signal.4. A microcontroller-based system for detecting ground-fault andgrounded-neutral conditions in an electrical power distribution systemhaving line and neutral conductors comprising: a sensor circuitproviding an output signal, said sensor circuit containing currenttransformer which has an inductance that varies with temperature and aresonant circuit, a programmable microcontroller containing apre-determined ground-fault threshold value and a pre-determinedgrounded-neutral threshold value stored in a non-volatile memory, saidmicrocontroller being programmed to initiate a ping signal to produce aresonant oscillation in said sensor resonant circuit during agrounded-neutral test, measure the frequency of said resonantoscillation to determine a change in the inductance of said currenttransformer, calculate a modified ground-fault threshold value based onsaid predetermined ground-fault threshold value and said change in theinductance of said current transformer, calculate a modifiedground-fault threshold value based on said grounded-neutral thresholdvalue and said change in the inductance of said current transformer, usesaid modified ground-fault threshold value to detect said ground-faultcondition, use said modified grounded-neutral threshold value to detectsaid grounded-neutral condition, and initiate the generation of a tripsignal upon detection of said ground-fault or said grounded-neutralcondition in said power distribution system, and a circuit interrupterfor interrupting current flow in said power distribution system inresponse to said trip signal.
 5. A method of detecting ground-fault andgrounded-neutral conditions in an electrical power distribution systemhaving line and neutral conductors, comprising: producing a signal witha single current transformer, responsive to current flow in both theline and neutral conductors of the electrical power distribution system,supplying said signal to a microcontroller that is programmed to usesaid signal to detect ground-fault or grounded-neutral conditions insaid power distribution system and initiate the generation of a tripsignal upon detection of said ground-fault or grounded-neutralcondition, interrupting the current flow in said power distributionsystem in response to said trip signal, and using an analog memory toprovide a timing function to control intervals for testing forground-fault or grounded-neutral conditions, and a memory function setin response to detection of a ground-fault or grounded-neutral conditionto resume a trip condition if power is temporarily lost before saidcurrent flow in said power distribution system is interrupted.
 6. Amethod of detecting ground-fault and grounded-neutral conditions in anelectrical power distribution system having line and neutral conductors,comprising: producing a signal with a single current transformer, thatvaries non-linearly over temperature, which is responsive to currentflow in both line and neutral conductors of said electrical powerdistribution system, and supplying said signal to a microcontroller thatis programmed during manufacture to receive said signal at a referencetemperature and calculate a predetermined ground-fault threshold valuebased on said reference temperature, and store said predeterminedground-fault threshold value in a non-volatile memory associated withsaid microcontroller, and receive said signal at a reference temperatureand calculate a predetermined grounded-neutral threshold value based onsaid temperature reference, and store said predeterminedgrounded-neutral threshold value in a non-volatile memory associatedwith said microcontroller.
 7. A method of detecting ground-fault andgrounded-neutral conditions in an electrical power distribution systemhaving line and neutral conductors, comprising; producing a signal witha sensor, which varies non-linearly with temperature, responsive tocurrent flow in both the line and neutral conductors of the electricalpower distribution system, producing an ambient temperature reading ofsaid sensor, and supplying said signal to a microcontroller having apredetermined ground-fault value and a predetermined grounded-neutralthreshold value, said microcontroller being programmed to use saidambient temperature reading to calculate a modified ground-faultthreshold value based on said predetermined ground-fault thresholdvalue, use said ambient temperature reading to calculate a modifiedgrounded-neutral threshold value based on said predeterminedgrounded-neutral threshold value, use said signal to detect ground-faultconditions based on said modified ground-fault threshold value, use saidsignal to detect grounded-neutral conditions based on said modifiedgrounded-neutral threshold value, initiate the generation of a tripsignal upon detection of a ground-fault or grounded-neutral condition,and interrupt the current flow in said power distribution system inresponse to said trip signal.
 8. A method of detecting ground-fault andgrounded-neutral conditions in an electrical power distribution systemhaving line and neutral conductors, comprising: producing a signalresponsive to current flow in both the line and neutral conductors ofthe electrical power distribution system with a sensor containing aresonant circuit and a current transformer having an inductance thatvaries with temperature, supplying said signal to a microcontrollerhaving a pre-determined ground-fault threshold value and apre-determined grounded-neutral threshold value, and saidmicrocontroller being programmed to initiate a ping signal to produce adamped oscillation in a sensor output signal during a grounded-neutraltest, measure the frequency of said damped oscillation to determine achange in the inductance of said current transformer, calculate amodified ground-fault threshold value based on said predeterminedground-fault threshold value and said change in the inductance of saidcurrent transformer, calculate a modified ground-fault threshold valuebased on said grounded-neutral threshold value and said change in theinductance of said current transformer, use said modified ground-faultthreshold value to detect a ground-fault condition, use said modifiedgrounded-neutral threshold value to detect a grounded-neutral condition,and initiate the generation of a trip signal upon detection of saidground-fault or said grounded-neutral condition in said powerdistribution system.